Backscatter transmission is a radio technique whereby signals are sent with typically lower power consumption than comparative techniques. The system requires a Radio Frequency (RF) source, an antenna, a receiver, and a transmitter. Most radio systems include a transmitter and a receiver, both of which are coupled to a logic circuit. The source sends a radio wave over the air using the transmitter. The radio wave propagates from the transmitter's antenna to the receiver's antenna. The impedance terminating the receiver/transmitter's antenna can be in one of three general states: open, short, or the same impedance as the antenna's characteristic impedance. When the impedance characteristic of the antenna matches the characteristic input impedance of the antenna load, then the impedance is considered to be “the same” as the terms is used herein.
Referring now to FIG. 1, an antenna 10 is shown having a termination impedance characteristic that is representative of an open circuit or high impedance. Accordingly, the signal, having a specific electromagnetic wave property, propagates without change.
Referring now to FIG. 2, the antenna 10 is shown employed in a system 20 that has a characteristic termination impedance equal to the characteristic impedance of the antenna 10. Accordingly, the power reflected from the antenna is equal to the power absorbed. The characteristic impedance is created electronically by allowing a controlled current to flow through a diode 22. The impedance is then set to the desired value in response to the amount of direct current. Zo, the characteristic impedance, is set by the diode current as set forth in equation (1):
      Z    0    =            1              g        m              =                  KT        q                    I        DC                            K=Boltzman's constant        T=temperature in degrees K        q=electronic charge        
Referring now to FIG. 3, the antenna 10 is employed in a system 30 having a characteristic terminating impedance representative of a short or low impedance. Accordingly, the power reflected from the antenna 10 is approximately four times the reflected power value when connected to a system having a characteristic impedance that is the same as the antenna's characteristic impedance. The short is created with a significant amount of current flowing from IDC through the diode 32. The exact value of the short can be described and determined using equation (1) above.
A radio that uses the current art of backscatter requires that direct current be used to create the characteristic impedance and the short circuit. Such systems use power that shortens the battery life and generated a great deal of heat, which becomes a problem in design trends that dictate smaller and more compact components. Compact designs typically call for smaller batteries and reduced heat generation. Thus, what is needed is a system and method that minimizes, or even eliminates, current consumption in order to maximize battery life and reduce heat generation.